Description: This study focuses on a single health maintenance organization (HMO) offering both a traditional gatekeeper HMO and point of service (POS) plan. It will rely on a secondary analysis of 1994-95 United Health Care data, including files on enrollees, providers, and claims. Specifically, the study will use econometric analyses to identify: differences in PCP services and referral rates in the two settings; and differences in specialists' services in the two settings. The research will also help determine if more efficient specialists receive more PCP referrals in either plan. The project provides a conceptual framework based on cost sharing principles derived from standard demand models. Demand functions for each aspect of medical care are modeled, with specific attention to out-of-pocket expenditures. Substitution effects of self-referrals and PCP referrals will be considered, as well as the complementary effects of PCP-referrals and PCP care. The data will come from two unidentified HMOs in the Midwest (where managed care penetration is 15 percent). Only privately insured individuals will be included. The network of providers used is very extensive, with over 1,100 primary care physicians and 1,690 specialists. The network physicians are independent contractors and practice in three settings: solo or small single-specialty groups, multi-specialty groups, and large IPAs.